1. Field of the Invention
This invention relates to phase shifting and steering of high frequency electromagnetic signals.
2.Description of the Related Art
Electromagnetic signals are commonly guided from a radiating element to a destination via a coaxial cable, metal waveguide, or microstrip transmission line. As the frequency of the signal increases, these devices must have smaller cross-sections to transmit the signals. For example, a metal waveguide that is 58.420 cm wide and 29.210 high at its inside dimensions, transmits signals in the range of 0.32 to 0.49 GHz. A metal waveguide that is 0.711 cm wide and 0.356 cm high at its inside dimensions, transmits signals in the range of 26.40 to 40.00 GHz. [Dorf, The Electrical Engineering Handbook, Second Edition, Section 37.2, Page 946 (1997)]. As the signal frequencies continue to increase, a point is reached where use of these devices becomes impractical. They become too small and expensive, require precision machining to produce, and their insertion loss can become too great.
Frequencies exceeding approximately 100 GHz (referred to as millimeter waves) can be transmitted as a free-space beam. The signal from a radiating element is directed to a lens that focuses the signal into a millimeter wave beam having a diameter up to several centimeters. This form of transmission is referred to as “quasi-optic” when the lens diameter divided by the signal wavelength is in the range of approximately 1-10. In the optic regime, the lens diameter divided by the frequency wavelength is normally much greater than 10. [IEEE Press, Paul F. Goldsmith, Quasi-optic Systems, Chapter 1, Gaussian Beam Propagation and Applications (1999)]
One method of amplifying these high frequency beams is to combine the power output of many small amplifiers in a quasi-optic amplifier array. The amplifiers of the array are oriented in space such that the array can amplify a Gaussian beam of energy rather than amplifying a signal guided by a transmission line. However, commercial use of these “open” systems is not practical because they are fragile and can be contaminated by the surrounding environment. Also, there is no simple, durable and reliable mechanism for beam phase shifting or steering.
Conventional rectangular waveguides cannot be used. In addition to their size and insertion loss disadvantages they do not provide an optimal signal to drive an amplifier array. Because the sidewalls of a metal waveguide are conductive, they present a short circuit to the beam's E field and it cannot exist near the conductive sidewall. The power densities of the beam's E and H fields drop off closer to the sidewalls, with the power density of the beam varying from a maximum at the middle of the waveguide to zero at the sidewalls.
For an amplifier array to operate efficiently, each individual amplifier in the array must be driven by the same power level. When amplifying the type of signal provided by a conventional metal waveguide, the amplifiers at the center of the array will be overdriven before the edge amplifiers can be adequately driven. In addition, individual amplifiers in the array will see different source and load impedances depending upon their locations in the array. The array's edge amplifiers become ineffective, significantly reducing the array's potential output power.
A high impedance surface will appear as an open circuit and the E field will accordingly not experience the drop-off associated with a conductive surface. A photonic surface structure has been developed which exhibits a high impedance to a resonant frequency and a small bandwidth around that frequency [D. Sievenpiper,High Impedance Electromagnetic Surfaces, (1999) PhD Thesis, University of California, Los Angeles]. The surface structure comprises patches of conductive material mounted in a sheet of dielectric material, with conductive vias through the dielectric material from the patches to a continuous conductive layer on the opposite side of the dielectric material. This surface presents a high impedance to the resonant frequency and the gaps between the patches prevent surface current flow in any direction.
A second impedance structure has been developed that is particularly applicable to the sidewalls and/or top and bottom walls of metal rectangular waveguides. [M. Kim et al., A Rectangular TEM Waveguide with Photonic Crystal Walls for Excitation of Quasi-Optic Amplifiers, (1999) IEEE MTT-S, Archived on CDROM]. Either two or four of the waveguide's walls can have this structure, depending upon the polarizations of the signal being transmitted. The structure comprises parallel conductive strips on a substrate of dielectric material. It also includes conductive vias through the sheet to a conductive layer on the substrate's surface opposite the strips. At the resonant frequency, this structure presents as series of high impedance resonant L-C circuits.
When used on a rectangular waveguide's sidewalls, the structure provides a high impedance boundary condition for the resonant frequency's E field component for a vertically polarized signal, the E field being transverse to the conductive strips. The high impedance prevents the E field from dropping off near the waveguide's sidewalls, maintaining an E field of uniform density across the waveguide's cross-section. Current can flow down the waveguide's conductive top and bottom walls to support the signal's H field with uniform density. Accordingly, the signal maintains near uniform power density across the waveguide aperture.
When the high impedance structure is used on all four of the waveguide's walls, the waveguide can transmit independent cross-polarized signals with near-uniform power density. The structure on the waveguide's sidewalls presents a high impedance to the E field of the vertically polarized signal, while the structure on the waveguide's top and bottom walls presents a high impedance to the horizontally polarized signal. The structure also allows conduction through the strips to support the signal's H field component of both polarizations. Thus, a cross-polarized signal of uniform density can be transmitted.
Waveguides employing these high impedance structures are also able to transmit signals close to the resonant frequency that would otherwise be cut-off because of the waveguide's dimensions if all of the waveguide's walls were conductive. At the resonant frequency, the waveguide essentially has no cut-off frequency and can support uniform density signals when its width is reduced well below the width for which the frequency being transmitted would be cut-off in a metal waveguide.